An internal combustion engine includes a catalyst disposed in its exhaust gas passage for purifying exhaust gas. However, the catalysts may not be capable of sufficiently purifying the exhaust gas when the temperature in the exhaust gas is not sufficiently high during, for example, the cold start of the internal combustion engine. Therefore, a secondary air injection system has been proposed in which secondary air is introduced into the exhaust gas passage upstream of the catalyst by using a motor-driven air pump and an air switching valve to increase the concentration of oxygen in the exhaust gas and the air-fuel ratio thereof. Accordingly, the system promotes secondary combustion such as of HC and CO in the exhaust gas to thereby purify the exhaust gas and activate the catalyst at an earlier time by raising the temperature of the exhaust gas. This system is disclosed in, for example, JP-A-2003-138929 and JP-A-2003-201834, the contents of which are incorporated herein by reference. In the systems disclosed in these patent documents, an O2 sensor or an exhaust gas sensor is arranged in the exhaust gas passage of an internal combustion engine, and an electronic control unit (ECU) diagnoses an abnormal condition of the system upon receiving output signals from the sensors. In the system disclosed in JP-A-2003-201834, in particular, an output signal from a pressure sensor arranged between an air pump and an air switching valve is sent to the ECU to specify in which of the air pump or the air switching valve the fault has occurred.
For example, if an air pump becomes faulty and an over-current continues to flow into a drive motor thereof, it is probable that the system as a whole may be damaged. Therefore, it becomes necessary to take measures to readily discontinue operation of the drive motor. In the conventional systems, however, it is necessary to operate the entire system inclusive of the ECU for diagnosing abnormal condition. That is, a certain period of time is required until the function for diagnosing the abnormal condition is operated by the ECU, thereby making it difficult to cope with the trouble in a short period of time and causing damage to the system.
Further, the conventional system utilizes the negative pressure on the intake side of the internal combustion engine to operate the air switching valve. Therefore, in a region of a high altitude where the atmospheric pressure is low, the air switching valve may not work to a sufficient degree. Moreover, an electromagnetic valve operatively controlled by the ECU is separately needed for introducing the negative pressure to the air switching valve, thereby resulting in a complex system constitution. An electromagnetic valve instead of the air switching valve may be used to deal with the problem associated with use of the air switching valve, and means may be provided to control the electromagnetic valve together with the air pump relying upon the ECU as taught in, for example, JP-A-7-11939 entitled “Device for Purifying Exhaust Gas of Diesel Engines”, the contents of which are also incorporated herein by reference. According to this means, a semiconductor switching element for driving the electromagnetic valve, which is driven by a small current, can be incorporated on the side of the ECU. However, the switching element for driving the air pump requires a capability of permitting a rush current of about 100 to about 200 A. If its heat-radiating structure and the handling of the wire harness inclusive of the connectors are taken into consideration, the semiconductor switching element for driving the air pump must be constructed separately from the ECU. Therefore, there inevitably occur duplicated portions in the power source circuit unit and in the protection circuit unit in their respective drive circuit blocks, thereby leading to higher production costs.